The present invention relates to a power transmission belt, namely a V-belt or V-ribbed belt, in particular of K type, for motor vehicle applications.
The ribs of such belts, which are composed of vulcanized rubber, have faces in direct contact with the crankshaft pulley and with the pulleys of the driven accessories.
Internal combustion engines exhibit a phenomenon of cyclic disturbance which becomes more pronounced as the compression ratio increases (in particular in diesel engines) and as the number of cylinders is reduced. This cyclic disturbance imposes significant mechanical stresses on the belts.
Some accessories, for example a charging alternator, exhibit high inertias, which generates levels of dynamic torques which increase in significance as the cyclic disturbance increases. These variations in the level of torque are reflected in their turn by large, indeed even very large, differences in tension of the belt.
When the level of tension of the belt is too low, it can exhibit momentary sliding which is sufficiently great to prevent driving and to generate noise.
The belt must thus exhibit a coefficient of friction sufficient to prevent this phenomenon.
On the other hand, if the coefficient of friction of the belt is too high, a phenomenon of significant sticking in the receiving pulley, followed by sudden slipping, which generates noise (stick-slip), may be observed.
A third possible cause of noisiness is the relative misalignment of two pulleys, which also generates a noise-generating stick-slip phenomenon which increases in significance as the coefficient of friction increases. This phenomenon also depends on the span length, on the dimensions of the belt and on the nature of the materials of which it is composed.
It is known, in order to find a solution for the first two causes (excessively low or excessively high coefficient of friction), to reduce the variations in tension by the addition of devices, such as a damper on the crankshaft (AVT) or else of a free wheel or of a decoupler pulley on the alternator.
It is also possible to increase the mean level of tension of the belt but this solution only partially solves the problem.
Finally, these solutions do not make it possible at all to put right a very great misalignment and to solve the noise phenomenon which is related to it.
It is therefore desirable to find a satisfactory solution for controlling the adhesion between the belts and the pulleys and which, if possible, avoids the addition of auxiliary devices, such as a damper on the crankshaft or else a free wheel or a decoupler pulley on the alternator.
Provision has been made, in French patent application FR 2 210 251, to limit the contact noise by depositing, on the toothing of a belt, a film of ultra-high molecular weight polyethylene or else by depositing a film of polyethylene only on the top of the toothing (PCT application WO 2004/011822), in order to prevent inadequate power transmission due to an excessively low coefficient of friction.
In both cases, shear and bending/reverse bending stresses are undergone by the continuous film, which results in damage to the film by cracking and splitting and thus in an inadequate lifetime of the coating.
Furthermore, provision has been made to incorporate, in the toothing, materials in the form of fillers, for example graphite in the case of patents U.S. Pat. No. 4,024,773, U.S. Pat. No. 4,031,768 or else U.S. Pat. No. 4,892,510.
These fillers are incorporated without sufficient bonding with the polymer of the toothing, which means that, during contact with sliding, filler components are torn off and the rubber/filler rubbing is interrupted. The result of these fillers being torn off is that the initiation of splitting is facilitated and the elastomer is locally weakened.
Thus, the materials deposited at the surface provided do not have an adhesion which is sufficiently strong to guarantee a coefficient of friction which is sufficiently stable during the lifetime of the belt. Because of abrasion, the coefficient of friction gradually increases, resulting in the gradual appearance of a phenomenon of noisiness.
It is an object of the present invention to provide a belt, the performance of which is improved as regards the phenomenon of noisiness.